This invention relates to the technical field of photoelectric reading of the image recorded on originals, used for photographic printers and image recording apparatus.
Images recorded on photographic film originals (hereunder referred to simply as "films") such as negative films and reversal films are conventionally printed onto photographic paper and other light-sensitive materials by "direct" exposure, or a method in which the recorded image of the film is projected onto the light-sensitive material for areal exposure.
A recent model under development is printers that rely upon "digital" exposure. Called a "digital photoprinter", this apparatus reads photoelectrically the image information recorded on a film, applies various steps of image processing to provide recording digital image information, records the image (latent image) by scan exposing the light-sensitive material with recording light modulated in accordance with the image information, and produces a print.
In digital photoprinters, the time required for the exposure per one image for the print is short and the exposure time in accordance with the image size is constant, since the film is photoelectrically read and subjected to the color/density corrections by means of signal processing to determine the exposure conditions. Therefore, the development can be performed more quickly than in the conventional areal exposure.
Further, it is easy to perform editing jobs such as synthesizing a plurality of images into a composite image or dividing a single image into segments and procedures of image processing such as color/density adjustments, and finished prints can be produced after desired editing and image processing has been made in accordance with a specific use. In addition, the image recorded in the finished print can be stored as image information in the external storage media such as floppy disks and this offers the advantage that extra printing and other jobs can be accomplished without any films that serve as the original. What is more, extra printing and other jobs can be performed in a rapid and efficient manner without repeating the setting of exposing conditions.
Furthermore, with prints produced by the conventional "direct" exposure, not all of the image recorded in films and the like can be reproduced in such aspects as resolution and color/density reproducibility. In contrast, digital photoprinters are capable of outputting prints that feature almost complete reproduction of the image (density information) recorded on the films.
The basic components of the digital photoprinter under consideration are an image reading apparatus for reading the image recorded on the film, a setup apparatus that performs image processing operations on the thus read image to determine the exposing conditions for image recording, and an image recording apparatus that performs development after a light-sensitive material was scan exposed in accordance with the determined exposing conditions.
The assignee invented various image reading apparatus and methods for realizing such digital photoprinters and proposed them in Unexamined Published Japanese Patent Application Nos. 217091/1994, 233052/1994 and 245062/1994, together with the disclosure of an outline of the digital photoprinter.
In the image inputting apparatus used in the digital photoprinter and other apparatus, the reading light emitted from a light source is applied to the original so as to obtain projecting light (or reflecting light) that bears the original image and which is then projected onto an image sensor such as a CCD (Charge Coupled Device) sensor in a sufficiently focused position to read the original image photoelectrically.
From an S/N (signal/noise) ratio view point, the image reading with the image sensor is preferably conducted with such a quantity of light that the output from the image sensor reading the brightest area of the image on an original or its nearby area is its saturation output.
In fact, however, the image density differs from one original to another and it is impossible to uniquely determine the quantity of light from the source that is appropriate for all kinds of original.
Under these circumstances, it is preferred for the image inputting apparatus to employ a light source which emits more light than is required to saturate the output of the image sensor in the absence of an original in the optical path, with a variable aperture stop being provided in the optical path from the light source to the image sensor and with image reading being performed by adjusting the degree of opening of the stop such that the sensor output corresponding to the brightest area of the image on the original approximates the saturation output of the sensor.
With the image inputting apparatus, main scanning for reading the image in order to produce the final output image is usually preceded by prescanning for rough image reading in order to determine the image processing and reading conditions and, hence, the degree of opening of the variable aperture stop in main scanning is determined in accordance with the image information obtained by the prescanning operation.
Needless to say, the output of the image sensor for the brightest area of the original image is preferably close to the saturation output of the image sensor even in the prescanning. However, as already mentioned, the image density varies from one original to another and it is difficult to determine the degree of opening of the variable aperture stop for the prescanning of an individual original; therefore, it is necessary to set a basic quantity of reading light, or illuminance, that is adapted for the reading apparatus.
In addition, the quantity of light from the source and the efficiency of optics (optical devices) usually involve individual differences from one reading apparatus to another and, hence, the setting of illuminance has to be performed for each apparatus. What is more, the quantity of light from the source varies with time due, for example, to aging and the deposit of dust particles and other foreign matter, so it is preferred to perform the setting of illuminance at each start-up of the reading apparatus. Further in addition, the sensitivity of the image sensor and the components of the reading light issuing from the light source usually differ for each of the three primary colors, red (R), green (G) and blue (B); therefore, with an apparatus for reading color images, illuminance is preferably set for each of the three primaries.